WO 2011/148280 A1 discloses a device and a method for measuring an analyte of a subject, the device comprising:
a number of narrow band light sources, each narrow band light source being structured to emit a spectrum of light covering a number of wavelengths; and
a number of detector assemblies configured to receive light reflected from a subject, each of the detector assemblies including a filter and a photodetector, each filter being structured to transmit a main transmission band and one or more transmission side bands, wherein for each narrow band light source the spectrum thereof includes one or more wavelengths that fall within the one or more transmission sidebands of any of the filters.
The document further discloses several refinements of the method and the device. For instance, it is suggested to utilize respective light emitting diodes (LED) as the narrow band light sources. Furthermore, it is envisaged to integrate both the narrow band light sources and the detector assemblies into a single system and to position the integrated system closely to a measurement surface of a subject to be monitored. Eventually, the document seeks after a determination of transcutaneous bilirubin and, based thereon, an estimation of a serum bilirubin level.
While basically avoiding blood sampling for assessing a subject's physiological condition or health condition, the device and method of WO 2011/148280 A1 may still be considered as an obtrusive approach for subject monitoring or patient monitoring, at least to a certain extent. The teaching of WO 2011/148280 A1 pertains to the field of contact measurement and/or contact monitoring basically requiring to closely attach sensors, emitters, transducers and further equipment to the monitored subject. This may be experienced as being considerably unpleasant. Particularly this holds true in the field of neonatal monitoring or, more generally, infant monitoring.
Recently, remote digital image-based monitoring systems for obtaining patient information or, physiological information of living beings in general, have been described and demonstrated.
As used herein, the term “remotely detected electromagnetic radiation” may refer to radiation components which are sent to a subject of interest from a radiation source (such as a remotely positioned light source) and “reflected” by a skin portion or dermal portion of the subject of interest. Also the subject's tissue beneath the skin's top surface plays a role in the reflection, deflection and/or absorption of incident radiation. Since reflection mechanisms in the subject's skin are rather complex and multi-dependent on factors such as wavelengths, penetration, depth, skin composition, vascular system structure, and further influencing parameters, terms such as “emitted”, “transmitted” and “reflected” shall not be understood in a limited way. Typically, a portion of incident radiation may be reflected at the skin's (upper) surface. Furthermore, a portion of incident radiation may penetrate the skin and pass through skin layers. Eventually, at least a portion of the incident penetrating radiation may be absorbed in the skin, while at least another portion of incident penetrating radiation may be scattered in the skin (rather than reflected at the skin's surface). Consequently, radiation components representing the subject of interest which can be captured by a sensor, particularly an image sensor, can be referred to a re-emitted radiation in this context.
For remote monitoring and measurement approaches, the use of cameras has been demonstrated. Cameras may particularly involve video cameras capable of capturing sequences of image frames. Preferably, cameras capable of capturing visible light can be used. These cameras may comprise a certain responsibility (or: sensitivity) characteristic which covers at least a considerable portion of a visible light range of the electromagnetic spectrum. As used herein, visible light shall be understood as part of the electromagnetic spectrum which can be sensed by the human eye without further technical aids.
Remote subject monitoring, e.g., patient monitoring, is considered beneficial since in this way unobtrusive non-contact measurements can be conducted. By contrast, non-remote (contact) measurements typically require sensors and even markers to be applied to a skin portion of interest of the subject to be monitored. In many cases, this is considered unpleasant, particularly for long-term monitoring.
It would be therefore beneficial to provide for a system and a method for remote monitoring which further contribute to overcoming the need of obtrusive (contact) measurements.
Photoplethysmography (PPG) is an optical measurement technique that evaluates a time-variant change of light reflectance or transmission of an area or volume of interest. PPG is based on the principle that blood absorbs light stronger than surrounding tissue, so variations in blood volume with every heartbeat affect transmission or reflectance correspondingly. Besides information about the heart rate, a PPG waveform can comprise information at reputable further physiological phenomena such as respiration.
In this connection, Verkruysse et al., “Remote plethysmographic imaging using ambient light”, Optics Express, 16(26), 22 Dec. 2008, pp. 21434-21445 demonstrates that photoplethysmographic signals can be measured remotely with normal ambient light and rather conventional consumer level video cameras.
Conventional PPG devices, such as pulse oximeters for measuring the heart rate and the (arterial) blood oxygen saturation (also called SpO2) of a subject are to be attached to the skin of the subject, for instance to a finger tip, earlobe or forehead. Therefore, they are referred to as “contact” PPG devices.